Butyl rubber and related polymers are generally derived from at least one isoolefin monomer and at least one copolymerizable monomer. Commercial butyl rubbers comprise a major portion of isoolefin and a minor amount of a conjugated multiolefin. One example of butyl rubber is poly(isobutylene-co-isoprene), or IIR, which has been prepared since the 1940's through random cationic copolymerization of isobutylene with small amounts of isoprene. The backbone structure of IIR, which is mostly comprised of polyisobutylene segments, imparts superior air impermeability, oxidative stability and excellent fatigue resistance to this material (see, for example, Chu, C. Y. and Vukov, R., Macromolecules, 18, 1423-1430, 1985).
In the production of articles made of rubber, curatives are used to toughen or harden the rubber. To achieve an appropriate mechanical property, curatives, often in the form of small molecules, metal oxides, or metal ions, are added to uncured rubbers (this process is also known as compounding) followed by compression molding at an elevated temperature. During the compression molding process, the curatives react to form crosslinks between the polymer chains of the rubber leading to a cured rubber article with meaningful mechanical properties. Such properties include good tensile strength combined with a high elongation and an appropriate compression set.
Conventional cure systems include a sulfur cure, a zinc oxide cure and a peroxide cure. In all three systems the curatives, or byproducts thereof, remain in the cured rubber article. The curatives, or byproducts thereof, could potentially leach out of the article and contaminate the user or the surroundings of the cured rubber article. For example, sulfur-cured rubber articles contain organic or inorganic sulfides and peroxide-cured rubber articles contain alcohols or ketones (and potentially unreacted coagents) that pose the risk of leaching and contaminating the adjacent media. This risk is particularly relevant to the variety of pharmaceutical and consumer goods applications of cured rubber articles (i.e. the sterile solution next to a pharmaceutical stopper or the stopper part of a syringe; the baby food next to the sealing ring of a jar lid).
Accordingly, there remains a need for a “clean” process for curing butyl rubber which reduces or eliminates the use of curatives in curing systems thereby reducing the potential for contaminant leaching.